1. Field of the Invention
The present invention relates generally to motor control systems. In particular, the invention is an improvement to a two-degree of freedom servo controller for use with a motor.
2. Description of the Prior Art
Closed-loop servo systems are commonly used to control motor position or velocity. Servo systems are typically defined by their system transfer function. The transfer function is the relationship of motor input to motor output. Internal operating parameters of the motor are elements of the system transfer function as are the transfer functions of feedback circuits, filters, tachometers and other components of the servo system.
Engineering analysis of motor control systems are normally made in the complex frequency domain. Laplace transforms are substituted for the input and output functions, as well as the internal operating parameters of the motor. These elements are then described as a function of a complex frequency s.
The command input to a velocity servo controller may be defined as a velocity command signal VCS(s). The controlled output of the system is the motor velocity w(s). The transfer function of the controller is defined as a ratio of the motor input to motor output, VCS(s)/w(s). Motor parameters of particular interest in the analysis include inductance L, resistance R, inertial load J, and a torque constant K.sub.T. K.sub.B represents the back EMF constant which is a physical parameter of the motor. Taken together, these parameters represent, in electrical terms, the operating characteristics of the motor.
The velocity servo controller utilizes feedback signals obtained from the motor. The first of these is a velocity feedback signal, K.sub.v w(s), produced by a tachometer connected to the output shaft of the motor. The constant K.sub.v represents a gain of the tachometer and is typically expressed in units of volts per RPM. A current feedback signal K.sub.i i(s) is also utilized. This signal may be taken directly from the motor or the drive amplifier. K.sub.i represents a gain of a transducer and is expressed in units of volts output per amp of drive current.
Two degree of freedom configurations for velocity servo control systems are well known. One of many such configurations in shown in FIG. 1. The velocity command signal VCS(s) and velocity feedback signal K.sub.v w(s) are summed at a first summing junction to produce an error signal. The error signal is processed by a first compensation circuit having a transfer function G.sub.v (s). The velocity feedback signal K.sub.v w(s) is processed by a second compensation circuit having a transfer function G.sub.f (s). The signals processed by the first and second compensation circuits are applied to a second summing junction, along with the current feedback signal K.sub.i i(s). The output of the second summing junction is applied to an amplifier having a gain K.sub.a. The output of the amplifier is the motor control voltage V.sub.m, which is applied to the motor itself.
By varying the transfer functions G.sub.v (s) or G.sub.f (s), it is possible to vary the bandwidth of the servo controller. The bandwidth is the range of frequencies over which the servo controller can respond. Changes in the transfer functions G.sub.f (s) or G.sub.v (s) also cause variations in a damping factor of the servo controller. The damping factor is a parameter indicative of the time it takes for the servo controller to settle to its steady state value after a change of the velocity command signal VCS(s) has been received. Although in actual operating environments it may often be desirable to vary both bandwidth and damping factor, prior art systems which vary both bandwidth and damping during tuning are difficult to work with. Additional variations may be introduced in response to other factors, such as load changes.
Both the bandwidth and damping factor are derived from the closed-loop transfer function of the servo controller and motor. These parameters contain common elements, however, and are therefore interrelated. Because conventional adjustments to either bandwidth or damping factor by readjusting G.sub.v or G.sub.f affect the other, no technique for decoupling these two parameters was known in the prior art. The tuning of bandwidth and damping factor is, therefore, an involved iterative process requiring extensive empirical manipulation.